Zimbabwe Craton - Ca. 2.7 Ngezi Group

Ca. 2.7 Ngezi Group

The Ngezi Group, ca. 2.7 Ga, of the Bulawayan Supergroup is found throughout the Zimbabwean Province. In the Belingwe Greenstone Belt of south central Zimbabwe, the Ngezi Group has been uplifted and eroded. Sedimentation and volcanism probably occurred here as a result of intracontinental extension associated with late Archaean plume.

The Ngezi Group consists of a basal sedimentary sequence (Manjeri Formation), overlain by komatiitic and tholeiitic volcanic rocks (Reliance and Zeedebergs Formations), and a second sedimentary sequence (Cheshire Formation). The late Archaean (~2.7 Ga) Ngezi Group greenstones in the Belingwe Greenstone Belt, south central Zimbabwe, show uplift and increased erosion during deposition of alluvial fans, local derivation of sedimentary material containing no arc derived component and eruption of komatiitic lava. Sedimentation and volcanism probably occurred as a result of intracontinental extension associated with an active, late Archaean plume.

The Manjeri Formation was deposited in a fluviatile and shallow marine setting, with subsequent alluvial fans and fan-deltas during active tectonism. Changes in the degree of chemical weathering of the provenance area during deposition of the formation, as measured by CIA values, reflect uplift and increased rates of erosion. Detrital mineralogy and rare-earth element patterns, are consistent with derivation from very local sources. Palæogeographic variation in the measured Sm-Nd depleted-mantle model ages between 2.9 and 3.7 Ga are consistent with deposition over basement varying in age from ca. 2.9 Ga - 3.5 Ga, again suggesting local derivation. The facies and geochemical association imply sedimentation in an extensional continental setting.

Samples by the NERCMAR drill hole through the 2.7 Ga Manjeri Formation in the Belingwe Greenstone Belt compared to data on the metamorphosed and deformed iron formations from the 3.7 Ga Isua Greenstone belt. Carbon and sulphur isotopic fractionations in the Belingwe samples may be interpreted in terms of a complex bacteria/archaea eclogical community. REE and Nd-isotopic variations may be modelled by contributions from a reduced hydrothermal component and a component surprisingly similar in REE pattern to modern seawater. Isua rocks are less well preserved but the overall similarity of the REE compositions implies deposition from a broadly similar ocean to that in the late Archaean.